mov: fix a possible invalid read in mov_read_mac_string()

When the input string is too large, so the second condition in if ()
fails, the code will erroneously execute the else branch, indexing the
mac_to_unicode table with a negative index.

CC: libav-stable@libav.org
Bug-Id: 1000
Found-By: Kamil Frankowicz

libavformat/mov.c

@@ -161,7 +161,11 @@ static int mov_read_mac_string(MOVContext *c, AVIOContext *pb, int len,
 
     for (i = 0; i < len; i++) {
         uint8_t t, c = avio_r8(pb);
-        if (c < 0x80 && p < end)
+
+        if (p >= end)
+            continue;
+
+        if (c < 0x80)
             *p++ = c;
         else
             PUT_UTF8(mac_to_unicode[c-0x80], t, if (p < end) *p++ = t;);